1. Field of the Invention
The present invention generally relates to an image processing apparatus, an image processing method, and a computer-readable encoding medium encoded with a computer program, and more particularly to the image processing apparatus, the image processing method, and the computer-readable encoding medium encoded with a computer program, in which a color conversion or a density correction is conducted.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating an example of image processes conducted by a printer driver, a controller of a printer, or a like. FIG. 1 illustrates that the image processes including a color conversion, a γ-correction, a halftoning process are conducted with respect to image data in which each gradation of an RGB (Red, Green, and Blue) is 8 bits for each pixel.
In the color conversion, a color space in which the image data are input is converted into another color space of a CMYK (Cyan, Magenta, Yellow, and Black) for an ink color. As a result, the image data in which each gradation of the CMYK is 8 bits for each pixel are output. In the γ-correction, a density correction is conducted, and the image data in which each gradation of the CMYK is 8 bits are output. In the halftoning process, a pseudo-continuous tone process is conducted by a dither method, a diffusion method, or a like, a gradation (8 bits) of the image data is converted into another gradation (for example, 2 bits) corresponding to a printer.
As illustrated in FIG. 1, there is a problem in that a gradation number of the output image is substantially deteriorated, if an information amount for depicting a gradation of an output image is lower than an information amount (8 bits) for depicting a gradation of an input image in a color conversion.
FIG. 2 is a diagram for explaining deterioration of the gradation number in the color conversion. FIG. 2 illustrates that input values (input image) are converted into output values (output image) by a color conversion using an LUT (Look Up Table) 510. In FIG. 2, a value of K (black) is “32” in an output value (lattice point value) with respect to an input value (lattice point) (0, 0, 0). Also, the value of K (black) is “28” in the output value with respect to an input value (0, 0, 16). Output values with respect to input values among lattice points are interpolated by conducting an interpolation calculation process such as a cubic interpolation, a triangular prism interpolation, a tetrahedron interpolation, or a like. Calculation results for K (black) by using the interpolation calculation process are exemplified as interpolation values in FIG. 2. However, if the information amount for depicting the gradation of the output value is the same as that of the input value, a difference with a value less than one (after the decimal point) can not be expressed. Accordingly, values defined as actual output values in FIG. 2 are applied as output values. As a result, 16 levels (0 through 16) of the gradation in the input values are reduced to 4 levels (32 through 28) of the gradation in the output values. As described above, if the information amount for depicting the gradation of the output image is fewer than the information amount of the input image, a resolution of an output side becomes lower than a lattice point interval, and the gradation number is substantially degraded.
Moreover, since a γ-correction parameter is generally curvilinear in shape, there is a problem in that when the information amount for depicting the gradation of the output image in the density correction of the γ-correction or a like is less than the information amount (8 bits in this example) for depicting the gradation of the input image, the gradation number of the output image is substantially degraded.
FIG. 3 is a diagram for explaining deterioration of the gradation number in the density correction. In FIG. 3, a table 520 shows input values and logical values. Also, the table 530 shows actual output values. As shown in table 530, if the information amount for depicting the gradation of the output image is less than the information amount for depicting the input image, it is not possible to depict a variance in values less than one in an ideal output value. As a result, the gradation number of the output value is deteriorated.
The deterioration of the gradation number described above causes discontinuity in a gradation image and causes a degraded image which is sufficiently recognizable by human eyes.